Liquid colorants for resins are known, as described in Japanese Patent Publication No. 33986/78 and Japanese Patent Application (OPI) No. 34348/79 (the term "OPI" as used herein means a "published unexamined Japanese patent application"). These liquid colorants have the following advantages.
(1) Usually, a powdered dry colorant composed of a pigment and a dispersant is preliminarily mixed with resin pellets in a tumbler or a mixer such as a Henschel mixer and then fed to a hopper of, e.g., an extrusion molding machine or an injection molding machine. In this case, therefore, such dry colorants are liable to cause pollution of environment due to the spattering and, furthermore, it is necessary to wash the hopper or mixer when changing the color of the colorants. However, it is known to add the liquid colorants are below the hopper. Thus they do not cause pollution of the environment and it is not necessary to wash the hopper or mixer when changing the color of the colorants.
(2) In accordance with another method, a dry colorant and a resin are mixed, the resulting mixture is compounded in a kneading extruder to prepare a colored compound, and then the colored compound is molded. This is called the "colored pellet method". When the liquid colorants are used, the coloring cost is low compared with the colored pellet method.
(3) In molding a master batch in which a pigment and a dispersant are dispersed in the resin both in high concentrations, a kneader having a high dispersion power such as a Banbury mixer is used. In this case, therefore, the amount of energy consumed is large. On the other hand, the liquid colorants can be kneaded with a kneader requiring a relatively low amount of energy, such as a three-roll mill or an "Attritor" (Trade name of Union Process Co.) (Szegvari Attritor). Thus, when the liquid colorants are used, the production cost is low.
In view of the above advantages, liquid colorants have been increasingly used in the art, greatly contributing to economization of the coloring step.
In feeding a given amount of the liquid colorant below the hopper of the extrusion molding machine or injection molding machine, a diaphragm pump, a plunger pump, a roller pump, and so forth have generally been used. These pumps, however, are limited in their capability to transport liquids of all viscosities. Thus it is necessary to adjust the viscosity of the liquid colorant to 100 poises or less. For this reason, liquid materials which can be used as the dispersants are limited to those materials having a viscosity of several poises at ordinary temperature, such as liquid paraffins, plasticizers, and surface active agents. Liquid colorants prepared using such low viscosity liquid materials as the dispersants are sufficiently satisfactory for the purpose of merely coloring the resin. However, when the colored molded article is used under more severe conditions, the inherent physical properties of the resin used in the production of the molded article may not be held because the dispersant component is liquid. The ABS resins are used particularly as interior parts of the car because of their superior heat resistance. Since these parts are used in a considerably high temperature atmosphere, the thermal deformation and bleeding of the additives to the surface due to the addition of the liquid colorants are seriously accelerated.
Pumps used to feed the colorant have been improved. For example, a gear pump capable of transporting high viscosity liquids having a viscosity of 1,000 poises or more in a definite amount or flow rate is now commercially available.